Problem: Which of these numbers is prime? ${15,\ 45,\ 63,\ 73,\ 88}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 73 are 1 and 73. The factors of 88 are 1, 2, 4, 8, 11, 22, 44, and 88. Thus, 73 is a prime number.